The usual problem with survivorship clauses is when the estate of the first person to die exceeds the nil-rate band, and the estate of the second person to die does not contain enough value to use up his or her nil rate band. That is not quite the same as what you appear to be referring to and so a variation may therefore be unnecessary. Since the wife died first, her nil rate band will pass to her husband, to the extent it is unused.
There is a rule that a variation must be capable of taking effect ‘in reality’ at the date the variation is made so I don’t think the will can be varied in favour of someone who has died.
Paul Davies
DWF LLP